How do I avoid the capital placeholders in python's argparse module?

Question

There was a question that asked where they come from, and the accepted answer was a bunch of links to tutorials and source code. Explanation for argparse python modul behaviour: Where do the capital placeholders come from?

None of it was helpful to me, I want to either get rid of them, or know their purpose.

For example, a line like this:

parser.add_argument('-c', '--chunksize', type=int, help='chunk size in bits')

produces garbage like this:

optional arguments:
  -h, --help            show this help message and exit
  -c CHUNKSIZE, --chunksize CHUNKSIZE
                        chunk size in bits

and if I try with an empty metavar string:

parser.add_argument('-c', '--chunksize', metavar='', type=int, help='chunk size in bits')

I get a space after the comma:

optional arguments:
  -h, --help            show this help message and exit
  -c , --chunksize      chunk size in bits

I thought that `argparse.SUPPRESS` might do the trick, but apparently not... — mgilson, Jun 06 '13 at 17:21
What is your desired output? `-c chunksize, --chunksize chunksize` instead of `-c CHUNKSIZE, --chunkesize CHUNKSIZE`? Then a `metavar = ('chunksize')` will do that. No help-line at all for that argument? `help=argparse.SUPPRESS` — Nisan.H, Jun 06 '13 at 17:23
For what it's worth, I think that the default behavior is better -- It's more explicit about which options take an argument (and how many). Of course, I'm not writing the program, so my input here may not matter (and that's fine) -- I'm just expressing my feelings on the matter. — mgilson, Jun 06 '13 at 17:23
@Nisan.H -- I think OP is looking for `-c, --chunksize ______ chunksize in bits` — mgilson, Jun 06 '13 at 17:24
Also, have a look at the MetavarTypeHelpFormatter at http://docs.python.org/dev/library/argparse.html#formatter-class and if that doesn't work, you can grab the source code and subclass your own formatter to format things the way you want them — Nisan.H, Jun 06 '13 at 17:27
I just noticed the MetavarTypeHelpFormatter is only available in 3.3 and up. I'll write up an answer for a custom formatter. — Nisan.H, Jun 06 '13 at 17:44

score 13 · Answer 1 · answered Jun 06 '13 at 17:18

13

parser.add_argument('-c', '--chunksize', metavar='\b', type=int, help='chunk size in bits')

seems to work

answered Jun 06 '13 at 17:18

Aravind

573
1
3
11

1

Neat hack, lol. I can't believe that's the right way to do this. – bug Jun 07 '13 at 12:41
1

you said it yourself; its a hack so can't be the right way, lol – Aravind Jun 10 '13 at 15:35
5

This will mess up the indention of option description – user1948847 Jun 04 '14 at 14:35

Nisan.H · Answer 2 · 2013-06-07T16:48:48.067

You can make your formatter class to format the arguments whichever way you want. It's not entirely straight forward, but here's one that produces the following output (assuming @mgilson is correct in the assumption that you wanted to only display the metavar once for the set of command names... Otherwise just specify an actual metavar='value' and it will display precisely that text.):

# without metavar specified:
-c, --chunksize CHUNKSIZE
                chunk size in bits
# with metavar specified:
-c, --chunksize some_metavar
                chunk size in bits

And the code for the class and reproducing the two outputs:

import argparse
# 2.7-3.2
class SingleMetavarHelpFormatter(argparse.HelpFormatter):
    def _format_action_invocation(self, action):
        if not action.option_strings:
            metavar, = self._metavar_formatter(action, action.dest)(1)
            return metavar

        else:
            parts = []

            # if the Optional doesn't take a value, format is:
            #    -s, --long
            if action.nargs == 0:
                parts.extend(action.option_strings)

            # if the Optional takes a value, format is:
            #    -s ARGS, --long ARGS
            else:
                default = action.dest.upper()
                args_string = self._format_args(action, default)

                ## THIS IS THE PART REPLACED
                #~ for option_string in action.option_strings:
                    #~ parts.append('%s %s' % (option_string, args_string)) ### this is change
                ## /SECTION REPLACED

                ## NEW CODE:
                parts.extend(action.option_strings)
                parts[-1] += ' %s' % args_string
                ## /NEW CODE
            return ', '.join(parts)


parser = argparse.ArgumentParser(
    prog='PROG',
    formatter_class=SingleMetavarHelpFormatter
    )

parser.add_argument('-c', '--chunksize', type=int, help='no metavar specified')
parser.add_argument('-w', '--with_metavar', type=int, help='metavar specified', metavar='some_metavar')

parser.print_help()

edit: To not show a metavar at all, you can pass an empty string to metavar:

parser.add_argument('-e', '--with_empty_metavar', type=int, help='empty metavar specified', metavar='')

The difference between doing that with the original class and the new class is the lack extra space character after the short command syntax.

#usage: PROG [-h] [-c CHUNKSIZE] [-w some_metavar] [-e]
#
#optional arguments:
#  -h, --help            show this help message and exit
#  -c CHUNKSIZE, --chunksize CHUNKSIZE
#                        no metavar specified
#  -w some_metavar, --with_metavar some_metavar
#                        metavar specified
#  -e, --with_empty_metavar
#                        empty metavar specified

But why would I want to have this metavar to begin with? If I look at typical help messages in Linux command line apps, I do not see this metavar line. — bug, Jun 07 '13 at 12:39
If you pass an empty string as the metavar, then that's what would be displayed. I included it in the edit. — Nisan.H, Jun 07 '13 at 16:47
Thanks, so I could write my own formatter to make this work. I still don't get why this has to be so cumbersome and why it isn't the default behaviour. — bug, Jun 10 '13 at 10:51
Without any particular background into the development of this module or the standards of help messages in linux command line applications, it seems pretty natural to me that a command that takes a parameter would display some placeholder parameter to denote its position (and sometimes type)... For what its worth, this is why I like Python so much--it's really easy to change such built-in behaviour to suit your own needs. — Nisan.H, Jun 10 '13 at 17:01

score 1 · Answer 3 · answered May 20 '21 at 21:58

Regarding your question about the purpose of uppercase metavar - the documentation says:

A single optional argument --foo that should be followed by a single command-line argument will be referred to as FOO.

This is the explanation, but I had to read it many times to realize that it simply means that FOO is a reminder to supply an argument, where the value of the argument=FOO. So using a more helpful example than foo:

# --days DAYS   <-- from an imaginary help page
--days 365  # for a year; DAYS=365
--days 7    # for a week; DAYS=7

How do I avoid the capital placeholders in python's argparse module?

3 Answers3

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